If you want to condemn sb, don't worry about the pretext (idiom, from Zuozhuan 左傳|左传); one can always trump up a charge against sb/Give a dog a bad name, then hang him.
Related Sentences
欲加之罪，何患无辞 ( If you want to add to your sin )
欲加之罪，何患无辞！ ( There is nothing wrong with wanting to add sin! )